1. Field of the Invention
This invention relates to an absorption photometer, and particularly to an absorption photometer suitable for the photometric analysis in which a small amount of sample is fed through a flow cell and the transmitted light is measured.
2. Description of the Prior Art
Absorption photometers are used in various analytical instruments. Among those instruments, typical ones which implement the measurement by feeding a liquid sample through a flow cell are the liquid chromatograph analyzer, flow injection analyzer, and automatic biochemical analyzer for clinic. The flow injection analysis is described, for example, in U.S. Pat. No. 4,557,601, which is hereby incorporated by reference.
For the measurement of a sample which is fed through a flow cell, it is important to know the composition (or variation of composition) of the sample which is being fed through the flow cell at that time point. Samples for measurement are given as solutions in carriers such as water or organic solvent. The measuring wavelength is set to the typical absorption peak of one compound and the variation of absorption at that wavelength is measured as a function of time. In case there is no absorption, the amount of light incident to the detector must have been known in order to attain a sufficient measurement accuracy. The amount of incident light to the detector varies depending on the variation in the power voltage, ambient temperature and light source position. For compensating the light source variations, a beam splitter is placed between the light source and flow cell to monitor the output light. However, such a monitoring method does not completely eliminate the variation of base line in the measurement result. For example, during the absorbance or optical density measurement, a signal indicating as if the absorption increases or decreases is generated even though there is no change in the sample. Accordingly, it is desired to perform the measurement which provides a signal solely based on the true absorption of the sample.